Synthesis and applications of soluble pentacene precursors and related compounds

ABSTRACT

The present disclosure relates to methods and systems for synthesis of bridged-hydropentacene, hydroanthracene and hydrotetracene from the precursor compounds pentacene derivatives, tetracene derivatives, and anthracene derivatives. The invention further relates to methods and systems for forming thin films for use in electrically conductive assemblies, such as semiconductors or photovoltaic devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/395,920 filed on Mar. 2, 2009 which claims the benefit of U.S.Provisional Application No. 61/033,466, filed on Mar. 4, 2008. Theentire disclosures of the above applications are incorporated herein byreference.

FIELD

The present disclosure relates to methods and systems for synthesis ofpentacene precursors and other oligoacene precursors, such asketone-bridged hydropentacenes having at least one of a 1,4-, 5,14-, or6,13-bridge and their corresponding ketals, which can be used insolution processes to efficiently form thin films containing pentaceneor other oligoacene(s). Also disclosed are methods and systems forforming such thin films for use in electrically conductive assemblies,such as semiconductors or photovoltaic devices.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Pentacene is recognized as a good organic semi-conducting material foruse in p-type organic thin film transistors (OTFT), due to its highcharge mobility. It is currently known that pentacene can be prepared byvolatile decarbonylation of a 6,13-bridged-6,13-dihydropentaceneprecursor, e.g., by heating to 150° C. to liberate carbon monoxide. Thisprecursor is reported to have some degree of solubility in a limitednumber of organic solvents such as chloroform and might therefore beamenable to spin coating using multiple castings, e.g., coats.

The characteristics of pentacene can be attributed to its high degree ofcrystallinity in the solid state. However, the high tendency ofcrystallization also renders its low degree of solubility in organicsolvents. Therefore it becomes difficult to prepare thin films throughsolution processes. In an effort to address this shortcoming, scientistsat Philips and IBM have developed a strategy of preparing precursors ofpentacene that are soluble in organic solvents, and are able to producepentacene after the films are prepared. The structures of pentaceneprecursor are usually cyclo-adducts of pentacene itself (acting as adiene) with another small volatile fragment (acting as a dienophile).After a film is produced by spin coating processes, relatively purepentacene can be regenerated upon heating through a retro-cyclizationprocess.

Since the purity of pentacene is crucial to the conductivity of films,the volatile fragment(s), which is released during the thermal process,should be expelled out of the crystal lattice as much as possible. Theyield of retro-cyclization must be high, and the smaller the leavingfragment the better. Heavy elements such as sulfur and chlorine, whichhave been present in many leaving groups of previous examples, are knownto readily contaminate the resultant films. The preparation of apentacene precursor, i.e. compound 1, which extrudes a unit of carbonmonoxide at 150° C., has been described by Chen et al. The small sizeand inert nature of carbon monoxide render it the best candidate forserving as a leaving fragment. The pentacene film thus producedexhibited typical OTFT characteristics, i.e. an on/off current ratioabout 1.2×10⁵ and field-effect mobility μ close to 0.01 cm²V⁻¹s⁻¹.

The solubility of compound 1 is a relatively low ˜0.7 mg/mL in somecommon organic solvents, e.g. methylene chloride and THF. However, inorder to prepare a film with required thickness therefrom, repeatedcasting processes are necessary.

However, it would be advantageous to provide a pentacene precursorhaving improved solubility in various organic solvents, as well as beingcapable of conversion to pentacene below 360° C. in a step that leaveslittle, it any, non-pentacene residue in the product. This would permitpentacene to be used in a larger range of commercial applications.Ideally, pentacene processes should be highly soluble in organicsolvents such that a limited number of castings are required to obtainuseful end-product. In addition to improved pentacene processes,anthracene and tetracene type-oligoacenes useful in organicsemiconductor and/or photovoltaic applications are also disclosed andshall be readily processable in accordance with the teachings of thepresent invention.

SUMMARY

Various embodiments hereof provide general methods for the synthesis ofbridged-hydropentacene, hydroanthracene and hydrotetracene. Theprecursor compounds disclosed herein can be referred to as: (1)pentacene derivatives, (2) tetracene derivatives, and (3) anthracenederivatives. Various embodiments hereof also include methods for carbonmonoxide expulsion from aromatic hydrocarbon derivatives by both thermaland photochemical routes; methods for device fabrication therefrom; anddevices prepared thereby, including semiconductor and photovoltaicdevices; and methods of preparing, from the precursors, oligoacenes foruse in conductive compositions, dye compositions, and others.

In some embodiments, the present description provides a general methodfor the synthesis of 1,4- and/or 5,14- and/or 6,13-bridgedhydropentacenes useful as soluble precursors for formation of pentaceneor substituted pentacenes, e.g., halopentacenes. In exemplaryembodiments, the present subject matter provides improved syntheticmethods for preparing bridged pentacene precursor compounds that containat least one: 1,4-dihydro-1,4-(ketone or ketal) bridge;5,14-dihydro-5,14-(ketone or ketal) bridge; 6,13-dihydro-6,13-(ketone orketal) bridge; but not limited to one bridge; as well as novel compoundssynthesized thereby. In various embodiments, the bridges can be ketoneor ketal groups having from one to four carbon atoms (i.e., C1-C4). Forketone-bridges, a C1 ketone (i.e. oxomethylene) group is consideredparticularly useful. The oxomethylene group of a bridge hereof can alsobe referred to as a carbonyl group.

Thus, in various embodiments, synthetic methods hereof can be employedto produce pentacene precursor compounds, such as:1,4-dihydro-1,4-oxomethylene-bridged pentacene;5,14-dihydro-5,14-oxomethylene-bridged pentacene;5,7,12,14-tetrahydro-5,14:7,12-di(oxomethylene-bridged)pentacene;1,4,8,11-tetrahydro-1,4:8,11-di(oxomethylene-bridged)pentacene;1,4,6,8,11,13-hexahydro-1,4:6,13:8,11-tri(oxomethylene-bridged)pentacene;ketal-bridged compounds corresponding to any of theseoxomethylene-bridged compounds; and halogenated versions of any of theforegoing, useful as halopentacene precursor compounds. Pentaceneprecursor compounds further include those illustrated in the variousschemes shown below as well as halogenated versions thereof.

Reference herein to ketal-bridged compounds corresponding to such ketone(e.g., oxomethylene)-bridged pentacene precursor compounds indicatesthose compounds in which an oxygen atom and a carbon atom correspondingto those of an oxomethylene oxo group of the latter compound participatein either ketal (i.e. bis C1-C4 alkyl ether) or C1-C4 spiroketalformation, whereby the ketal group forms the bridge that corresponds tothe oxomethylene bridge. Representative examples of such ketal bridginggroups include: dimethyl ketal; ethylene spiroketal, i.e.(1,2-ethanediyl)spiroketal; and butylene spiroketal, i.e.(2,3-butanediyl)spiroketal groups. The spiroketals can be formed fromC1-C4 diols, such as ethylene glycol or 2,3-butanediol. As used herein,“C1-C4 ketal group” refers to bis(C1-C4 alkyl) ketal groups and to C1-C4spiroketal groups. Ketals include acetals derived from ketones byreplacement of the oxo group by two hydrocarbyloxy groups R₂C(OR)₂,where R≠H. Spiroketals include one carbon atom as the only common memberof two rings.

In some embodiments hereof, halopentacene precursor compounds can beprovided that have the structure of any of the aboveoxomethylene-bridged or ketal-bridged compounds and are furtherhalogenated, e.g., chlorinated. Symmetrically halogenated halopentaceneprecursors are particularly useful in some embodiments. For example,2,3,9,10-tetrahalo and 1,2,3,4,8,9,10,11-octahalo forms can be used,such as 2,3,9,10-tetrachloro or 1,2,3,4,8,9,10,11-octachloro precursorcompounds. In some embodiments, halopentacene precursors can bedehalogenated to form pentacene precursors, or can be used to formhalopentacenes that can in turn be dehalogenated to form pentacenes. Anydehalogenation treatments known as useful therefor in the art can beemployed for this, e.g., treatment with sodium in tert-butanol andaromatization by dichloro dicyano quinone (DDQ) treatment.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent with color drawings will be provided by thePatent and Trademark Office upon request and payment of necessary fee.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 presents UV absorptions of compounds 1 (blue), 2 (red), and athermally generated pentacene (black). The long wavelength bands aremagnified to the center of graph.

FIG. 2 presents thermogravimetric analysis curves for compounds 1 (blue)and 2 (red) at 10° C./min heating rate, assessing the carbon monoxide(CO) expulsion reaction, in which the carbonyl bridge is released fromthe precursor for conversion to pentacene. The CO expulsion reaction of2 started at 128° C., better than the 150° C. for the start of reactionfor 1, and the resulting pentacene was stable till 300° C.

FIG. 3A presents absorption spectra of 2, in degassed tetrahydrofuran(THF), obtained upon irradiation with 366 nm UV light with differentexposure times, 0 to 40 sec, with increments of 5 sec. FIG. 3B presentscomparable absorption spectra of 1, in degassed THF, irradiated at 310nm as a function of exposure times of 0 to 40 sec, with increments of 5sec.

FIG. 4 presents plots of drain current I_(D) versus drain-source voltage(V_(DS)) at various gate voltages (V_(GS)) obtained from an OTFT with achannel width of 20 cm and a channel length of 10 μm.

FIG. 5 presents plots of log(I_(D)) vs. V_(GS) for V_(Ds)=−80V, and✓I_(D) vs. V_(GS) in the saturation.

FIG. 6 presents results for a dimethyl ketal-bridged pentaceneprecursor.

FIG. 7 presents results for a dimethyl ketal-bridged halopentaceneprecursor.

FIG. 8 presents an X-ray diffraction structure for compound 16.

FIG. 9 presents a thermogravimetric analysis (TGA) plot for compound 16.

FIG. 10 presents an image of a film made of compound 1.

FIG. 11 presents an image of a web-like pentacene film, formed fromcompound 1, after it has been heated to its conversion temperature.

FIGS. 12A and 12B present, respectively, a schematic of the structureand a photographic image of the material of OTFT devices made fromcompounds 1 and 2.

FIGS. 13A and 13B present graphs of test results showing typicalfield-effect transistor (FET) characteristics of a device made fromcompound 1, which are: μ=8.8×10⁻³ cm² V⁻¹ s⁻¹ and on/off current ratio˜1.2×10⁵.

FIGS. 14A and 14B present graphs of test results showing typical FETcharacteristics of a device made from compound 2, which are: μ=1.2×10⁻³cm² V⁻¹ s⁻¹ with on/off current ratio ˜1.82×10⁴.

FIGS. 15A and 15B present graphs of test results showing typical FETcharacteristics of a device made from compound 16. Parameters obtainedfrom a device of channel width of 1 mm and channel length of 5 μm are:field-effect mobility μ=7.89×10⁻⁶ cm²/Vs, and estimated on/off currentratio=1.7×10³. FIG. 15A presents plots of drain current I_(D) versusdrain-source voltage V_(DS) at various gate voltages V_(GS) obtainedfrom an OTFT with a channel width of 1 mm and a channel length of 5 μm.15B presents plots of log(I_(D)) vs. V_(GS) for V_(DS)=−80 V, and ✓I_(D)vs. V_(GS) in the saturation mode. The device made from 13 performedworse than those made from 1 and 2, because at a higher annealtemperature the substrates tend to evaporate from the surface of thefilm. Therefore it was difficult to maintain an ideal film thickness.

FIG. 16 presents normalized UV absorptions of compounds 1 (green), 2(brown), 25 (blue), 23 (red), pentacene (black), and 26 (pink). Thepentacene and compound 26 were produced by acidic hydrolysis of 25 and23, respectively. The inset shows the characteristic long wavelengthabsorptions of pentacene (black) and 26 (pink).

FIG. 17 presents thermogravimetric analyses (TGA) of compound 23e(blue), 25 (black), and 25e (red) at 10° C./min heating rate. The weightlosses of 23e and 25e correspond correctly to the production ofpentacene upon expulsion of CO₂ and ethylene units. A betterfragmentation can be obtained by slowing down the heating rate to 0.13°C./min between 230-240° C. (green), so that the evaporation of pentacenecan also be slowed down.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Amongoligoacene precursors provided herein are precursors for anthracene (A),tetracene (B), and pentacene (C), whose respective structures are shownbelow.

In various embodiments, the precursors possess higher solubility thanpreviously known precursors for a given oligoacene. In some embodiments,the precursors hereof possess different bridging group chemistriesand/or bridging architectures than those of such previously knownoligoacene precursors. Precursors for pentacenes are described below,followed by descriptions of tetracene and anthracene precursors hereof.

In various embodiments hereof, relatively higher solubility of pentaceneand other oligoacene precursors available through the synthetic methodsdescribed herein, allows for the preparation of large films byspin-coating processes. Substantially pure pentacene and otheroligoacene film can thereby be generated either thermally orphotochemically. OTFT devices made from these materials exhibit typicalFET characteristics and are useful in any industry that uses organicthin film transistors (OTFT). Applications can be extended to organicintegrated circuits, active-matrix flat-panel displays (AMLEDs such asOLEDs), radio frequency identification (RF-ID), photovoltaic cells,chemical sensors, and other flexible electronics, by way of non-limitingexample.

Various embodiments of new compounds provided herein are soluble inorganic solvents, so that they can be processed into large films by,e.g., a spin-coating technique. The pentacene precursors are capable ofproducing pentacene either thermally or photochemically by extruding,i.e. releasing, volatile units of CO or CO₂, as can other precursorshereof to their oligoacenes.

In thermal routes, the pentacene precursor fragmentation temperature(from ambient to 240° C.) is well below the thermal decompositiontemperature of pentacene (360° C.), while in some cases thefragmentation can also be initiated by photo-irradiation. The productionyield is nearly quantitative, thus generating pentacene in very highpurity. The molecular fragments expelled from the precursor compoundsduring conversion to pentacene were small organic molecules that arechemically inert and highly volatile. The high purity of resultingpentacene films is evidenced in OTFT devices, which exhibit typicalthin-film transistor (TFT) characteristics.

Representative Pentacene Carbonyl Adducts.

Designing electronic devices made of organic materials is of intensivecurrent interest, and organic thin-film transistor (OTFT) is anessential part of it. The most well-known organic gate material for OTFTso far is pentacene, which is a p-type semiconductor with good chargemobility.

In order to obtain improved p-type semiconductors, it has beendiscovered that using pentacene precursors having off center bridging ofketone/carbonyl or ketal groups, such as that shown in compound 2, giverise to compositions having enhanced solubility over precursors formedfrom compound 1. It has been unexpectedly found that, even with only aslight increase of solubility, substantial improvement of the spincasting process can be obtained by compounds prepared according to thevarious embodiments hereof. Compound 2 can be synthesized as describedbelow.

The synthetic pathway can be depicted in Scheme 1A, which starts from a[4+2] cycloaddition of a furan derivative 3 (dihydroisonaphthofuran)with a benzonorbornadiene derivative 4(benzo-(7-isopropylidene)norbornadiene). The adduct 5 was obtained in72% yield, and possesses the basic skeleton of pentacene with five fusedrings. Its C_(2v) symmetrical geometry was affirmed by the presence of14 distinctive absorption signals in ¹³C NMR spectrum. The two methylenehydrogens appear in ¹H NMR spectrum as two separate doublets at δ 3.27and 3.57 with a large coupling constant (20 Hz). Aromatization of theforth ring of 5 was achieved by an oxidation with dicyanodichloroquinone(DDQ), while 6 was obtained in a quantitative yield. The three types ofmethinyl hydrogens exhibit three singlets at δ 1.99, 3.93, and 5.33 in¹H NMR field. Further dehydration/aromatization under the catalysis oftoluenesulfonic acid resulted to a complex mixture of compounds 7, 8 and9 in a total yield of 75%. Their relative ratio depends on theconcentration of acid as well as the time of reaction. The quaternarycarbon of 7, which bears the hydroxyl group, shows a singlet signal at δ71.5 in ¹³C NMR spectrum. Compounds 8 and 9 are believed to be thesecondary products deriving from 7. The optimal yield of 9, which wasconsidered to be a suitable precursor of 2, was less than 10%. Toimprove the yield toward desired product, compound 6 was first treatedwith meta-chloro-perbenzoic acid (m-CPBA) to form an epoxide, followedby an acid catalyzed rearrangement. The diol 10 was obtained in 74% asthe main product. The presence of a 1,2-diol was evidenced by a strongabsorption band at 3450-3550 cm⁻¹ in IR spectrum, and the two quaternarycarbon signals at δ 74.3 and 105.5 in ¹³C NMR spectrum. Oxidation of 10by iodobenzene diacetate (PhI(OAc)₂) cleaved the C—C bond to give thedesired ketone 2 in 76% yield. The carbonyl group shows a strongabsorption at 1782 cm⁻¹ in IR, and a low field peak at δ 192.2 in ¹³CNMR. The optimal overall yield from (3+4) to 2 was 40%.

Ketone 2 can also be made from 9 through ozonolysis in a mixed solvent(CH₂Cl₂:MeOH=2:1) at −35° C., followed by a reduction of the ozonideintermediate with dimethylsulfide, however, this method is not preferredfor the production of compound 2. The UV absorption spectrum of compound2 is significantly different from that of compound 1 (FIG. 1). A strongπ-π* transition (¹A→¹B) of compound 2 gives a strong peak at 268 nm,which is longer than the corresponding absorption of compound 1 at 244nm. This band is known to be longitudinal polarized and is red-shiftedwith the size of polyacenes. The difference in the spectra of compounds1 and 2 is readily understood by the size of their major chromophores,i.e. an anthracene moiety in compound 2 and a naphthalene in compound 1.A much weaker band of ¹A→¹L_(a) transition appears at 387 nm, alsosignificantly red-shifted to the corresponding one of compound 1 at 300nm. The latter band is partly overlapped with the ¹A→¹L_(b) transitionat 324 nm.

Compound 2 is stable at ambient temperature, while it begins todecompose at about 128° C. and turns to purplish. The fragmentationtemperature is a little lower than that of compound 1, which decomposesat 150° C. The thermal stability of compound 2 is slightly lower than 1.A 10% weight loss, as indicated in a thermogravimetric curve (FIG. 2),corresponds to the weight ratio of a CO unit. The pentacene thusproduced is stable up to 300° C. Its UV spectrum shows a strong peak at300 nm (¹B band for a π-π* transition), along with a characteristic longwavelength absorptions at 576 nm (¹L_(a) band). The spectrum isidentical to that obtained from authentic pentacene, indicating a veryhigh yield for thermal transformation. In IR region, the strongabsorption of carbonyl group at 1782 cm⁻¹ diminished completely afterheating. The solubility of compound 2 is similar to that of compound 1,i.e. ˜0.7 mg/mL in dichloromethane (DCM) and tetrahydrofuran (THF).

As to its photochemistry, compound 2 also exhibited salientphotocolorant behavior. Upon ultraviolet irradiation at the absorptionpeak of ˜366 nm in room temperature, degassed THF, compound 2 turnedfrom colorless to purplish. Spectroscopically, as shown in FIG. 3A, withan increase of exposure time, a new absorption band with the peakwavelength around 575 nm gradually increased. An isosbestic point wasobserved at ˜395 nm throughout the photolysis, indicating the existenceof only two moieties in THF, the reactant compound 2 and product. Sincethe spectral feature of the new band, including the vibronic progressionand the associated peak wavelengths, is identical with that of pentacene(Cf. FIGS. 1 and 3), the CO expulsion from compound 2 upon UV excitationto form pentacene is well justified. Note the UV induced compound2→pentacene reaction has to be performed free from O₂. Under aeration,instead of the pentacene formation, fragmentation occurred, as indicatedby the resulting broad, diffusive absorption band of <400 nm uponphotolysis in the aerated solution (not shown here).

The yield of photo-product, i.e. pentacene, was further analyzed in amore quantitative manner. In a prototypical experiment, after using a 10mW/cm² 385 nm Nd:YAG pumped Ti³⁺:Sapphire laser (LOTIS TII, LT-2211) toilluminate the 3 mL solution containing compound 2 (˜1.2×10⁻⁴ M in THF)under vigorous stirring for 40 sec, it was observed that the absorbanceat 575 nm increased from near zero to ˜0.43, corresponding to anincrease of ˜4.3×10⁻⁵M of pentacene. By taking the ratio of the numberof pentacene being produced versus number of photon being absorbed bycompound 2, the yield of pentacene production was then calculated to be16.2±1.0%. In fact, as depicted in FIG. 3B, similar photochemistry takesplace in compound 1 upon 310 nm irradiation in degassed THF, the resultof which was previously unrecognized. Based on the similar photolysisprotocol, the yield of pentacene from compound 1 was estimated to be12.6±0.6%, which is significantly lower than the results for compound 2.

OTFT devices were also fabricated using a heavily n-doped Si wafer asthe substrate and the gate, and SiO₂ was thermally grown to 2000 Å as agate insulator. On top of the SiO₂, gold was deposited lithographicallyto 30 nm as the source and drain electrodes. Compound 2 was spin-coatedrepeatedly over the structure to form a thin film, which was then heatedto produce pentacene. The output characteristics of a device withchannel width 20 cm and length 10 μm exhibited typical FETcharacteristics. The OTFT operates in the p-type enhancement mode andexhibits a hard saturation. A plot of drain current (I_(D)) versusdrain-source voltage (V_(DS)) at various gate voltages (V_(GS)) is shownin FIG. 4. The corresponding transfer characteristics, i.e. log(I_(D))vs. V_(GS) for V_(DS)=−80V, and ✓I_(D) vs. V_(GS) in the saturation modeis shown in FIG. 5. The OTFT exhibits an on/off current ratio of about1.82×10⁴, and the apparent field-effect mobility μ is estimated to be1.2×10⁻³ cm² V⁻¹ s⁻¹. Due to the limited solubility of compound 2, thethin films prepared by spin-coating process were not entirelycontiguous. Since the pentacene did not cover the whole area between thesource and the drain, the actual mobility shall be higher.

In summary, a new stable and soluble pentacene precursor 2 wassynthesized in 40% yield from the cycloaddition of a furan derivative 3and a benzonorbornadiene 4. Compound 2 releases a unit of CO uponheating at 128° C. or by irradiation with UV light (366 nm) to producepentacene in nearly quantitative yield. Thin films of pentacene wereprepared by spin-coating followed by thermal annealing, and found todisplay typical FET characteristics.

In addition, synthesis of dicarbonyl compound 12 may be performedaccording to the present disclosure. Compound 12 is stable at lowtemperature for a long period of time, and exhibits a higher solubilitythan either compound 1 or 2. Compound 12 is suitable for making thinfilms by spin-coating process, and can generate pure pentacene byheating and/or by the irradiation of UV light. At ambient temperature inorganic solvent compound 12 loses one unit of CO slowly to yield thecorresponding monocarbonyl compound 2.

The synthesis of compound 12 can be depicted in the following Scheme 1B.The first step was a cycloaddition between dichlorobenzoquinone A and8-dimethylisobenzofulvene. An elimination of HCl was followed to give anorange-colored chloroquinone B in 93% yield. In this reaction the8-dimethylisobenzofulvene was generated in situ by reactingbenzo(7-isopropylidene)norbornadiene with3,6-di(2′-pyridyl)-s-tetrazine. This reaction sequence can be repeatedonce again to fuse another moiety of 8-dimethylisobenzofulvene to givecompound C in 85% yield. Compound C existed in a mixture of syn and antiisomers. Reductive aromatization of the central ring of C was achievedby reacting with sodium borohydride, while D was obtained in 92% yield.Compound E was obtained by the esterification of D to a ditriflate (70%)with trifluoromethanesulfonic anhydride, followed by apalladium-catalyzed reduction with formic acid. A final oxidation of Ewith ozone in dichloromethane at −78° C. cleaved the C═C double bondssuccessfully in 90% yield. The carbonyl groups of 12 showed a strongabsorption in IR at 1793 cm⁻¹, and in ¹³C NMR a signal at δ 188.5. Thetotal yield from A to 12 was about 37%.

Synthesis of compound B in Scheme 1B. To a solution of compound A (177mg, 1.00 mmol) and benzo(7-isopropylidene)norbornadiene (182 mg, 1.00mmol) in chloroform (5 mL) was added slowly3,6-di(2′-pyridyl)-s-tetrazine (236 mg, 1.00 mmol) at room temperaturewith stirring. The resulting solution was stirred for three hours, to itthen was added triethylamine (0.40 mL, 3.0 mmol) and the mixture wasstirred with a magnetic bar for another two hours. The mixture waswashed with dilute sulfuric acid (3×10 ml, 5%), water and brinesuccessively in an ice both. The organic solution was evaporated invacuo, while a dark brown oil of compound B (275 mg, 93%) was collected.Mp 199-200° C.; ¹H NMR (400 MHz, CDCl₃): δ 7.35 (m, 2H), 7.01 (m, 2H),6.75 (s, 1H), 4.96 (d, J=1.6 Hz, 1H), 4.91 (s, 1H); ¹³C NMR (100 MHz,CDCl₃): δ 180.9, 175.3, 158.8, 158.4, 157.7, 146.9, 146.8, 143.2, 132.5,125.7, 122.4, 122.3, 109.3, 49.5, 49.1, 19.0.

Synthesis of compound C in Scheme 1B. A crude product of B (296 mg, 1.00mmol) was treated with benzo(7-isopropylidene)norbornadiene (182 mg,1.00 mmol) and 3,6-di(2′-pyridyl)-s-tetrazine (236 mg, 1.00 mmol)according to the same procedure as the previous step. For the basecatalyzed elimination, DBN (0.25 mL, 2.0 mmol) was used instead oftriethylamine. Product C was purified by silica gel chromatograph elutedwith hexane/ethyl acetate (5:1) to form orange solids (354 mg, 85%) as amixture of anti and syn isomers. Mp 280-282° C.; ¹H NMR (400 MHz,CDCl₃): δ 7.34 (dd, J=5.2, 3.2 Hz, 4H), 7.27 (dd, J=5.2, 3.2 Hz, 4H),7.00 (dd, J=5.2, 3.2 Hz, 4H), 6.93 (dd, J=5.2, 3.2 Hz, 4H), 4.83 (s,4H), 4.82 (s, 4H), 1.56 (s, 12H), 1.52 (s, 12H); ¹³C NMR (100 MHz,CDCl₃): δ 179.4, 159.0, 158.8, 157.0, 156.9, 147.6, 147.2, 125.4, 122.1,108.2, 107.8, 48.8, 18.9, 18.8; MS (EI⁺) m/z 416 (M⁺, 100%), 401 (50),392 (34), 300 (68); HRMS (m/z) calcd for C₃₀H₂₄O₂ 416.1779. Found:416.1776.

Synthesis of compound D in Scheme 1B. To a round bottom flask containingcompound C (416 mg, 1.00 mmol) in a mixed solvents of methanol (5 mL)and THF (2 mL) at 0° C. was added NaBH₄ (0.15 g, 4.0 mmol) slowly toavoid abrupt generation of hydrogen gas. After the addition the mixturewas stirred at 0° C. for another two hours, and then was quenched by theaddition of water (5 mL). The mixture was evaporated in vacuo and wasextracted with dichloromethane (30 mL). The organic layer was dried overanhydrous MgSO₄, filtered, and concentrated in vacuo again. The residuewas purified by a flash chromatograph eluted with hexane/dichloromethane(1:2) to give colorless solids of compound D (385 mg, 92%). ¹H NMR (400MHz, CDCl₃): δ 7.27 (dd, J=5.2, 3.2 Hz, 4H), 7.20 (dd, J=5.2, 3.2 Hz,4H), 6.93 (dd, J=5.2, 3.2 Hz, 4H), 6.87 (dd, J=5.2, 3.2 Hz, 4H), 4.84(s, 4H), 4.82 (s, 4H); MS (EI⁺) m/z 418 (M⁺, 100%), 403 (35), 390 (20);HRMS (m/z) calcd for C₃₀H₂₆O₂ 418.1935. Found: 418.1933.

Synthesis of compound E in Scheme 1B. To a solution of compound D (418mg, 1.00 mmol) and triethylamine (2 mL) in chloroform (20 mL) was addedtrifluoromethanesulfonic anhydride (1.08 g, 3.8 mmol) dropwise withstirring at 0° C. After the addition was completed, the mixture waswarmed up to room temperature and stirred for further 30 min. Thereaction mixture was poured into ice-water (40 mL), and was extractedwith ether. The organic layer was washed with aqueous HCl (2×20 mL, 1M),dried over MgSO₄, and concentrated in vacuo. The residue was purified bya silica gel column chromatograph eluted with hexane/dichloromethane(5:1). A triflate derivative of D was obtained (478 mg, 70%) as whitesolid. ¹H NMR (400 MHz, CDCl₃): δ 7.36 (dd, J=5.2, 3.2 Hz, 4H), 7.27(dd, J=5.2, 3.2 Hz, 4H), 7.03 (dd, J=5.2, 3.2 Hz, 4H), 6.96 (dd, J=5.2,3.2 Hz, 4H), 4.97 (s, 4H), 4.94 (s, 4H), 1.60 (s, 12H), 1.53 (s, 12H);¹³C NMR (100 MHz, CDCl₃): δ 157.5, 157.2, 147.4, 147.1, 143.1, 135.3,135.1, 125.8, 125.8, 123.5, 122.2, 122.1, 120.3, 117.1, 114.0, 110.3,110.0, 49.8, 19.1, 19.0; MS (EI⁺) m/z 682 (M⁺, 100%), 601 (18); HRMS(m/z) calcd for C₃₂H₂₄O₆ 682.0928. Found: 682.0919.

The triflate compound (682 mg, 1.00 mmol) was mixed with1,3-bis(diphenylphosphino)propane (0.16 g, 0.38 mmol),bis(triphenylphosphino)palladium(II) chloride (0.10 g, 0.15 mmol),formic acid (1.0 mL) in dimethyl formamide (10 mL), and tri-n-butylamine(2.5 mL) together. It was stirred with a magnetic bar at 110° C. under anitrogen atmosphere for 72 hours. After addition of aqueous HCl (30 mL,1.5 M), the mixture was extracted with dichloromethane (2×30 mL). Thecombined organic layers were washed with aqueous HCl (2×30 mL, 1 M),dried over MgSO₄, and concentrated in vacuo. The yellow residue waspurified by a silica gel column chromatograph eluted withhexane/dichloromethane (3:1) to yield compound E as colorless solids(274 mg, 82%). ¹H NMR (400 MHz, CDCl₃): δ 7.22 (m, 6H), 6.90 (dd, J=5.2,3.2 Hz, 4H), 4.61 (s, 4H), 1.50 (s, 12H); MS (EI⁺) m/z 386 (M⁺, 100%),371 (42); HRMS (m/z) calcd for C₃₀H₂₆ 386.2029. Found: 386.2035.

Synthesis of compound 12 in Scheme 1B. A two-necked flash fitted with aglass tube to admit ozone, and a CaCl₂ drying tube was charged withcompound E (1.00 g, 1.30 mmole) in anhydrous dichloromethane (30 mL).The flask was cooled to −78° C., and ozone was bubbled through thesolution with stirring. The stream of ozone was stopped as soon as thesolution turned bluish. Nitrogen gas was passed through the solutionuntil the blue color was discharged. To the resulting solution was addeddimethyl sulfide (1.0 mL, 1.3 mmol) at −55° C., and was stirred for sixhours. The mixture was concentrated in vacuo at 0° C. and products wereseparated quickly by a silica gel chromatograph maintained at −10° C.eluted with hexane/dichloromethane (1:2). Compound 12 (274 mg, 82%) wasobtained as pale yellow solids which can be kept at −10° C. for anextended period of time. ¹H NMR (400 MHz, CDCl₃): δ 7.59 (s, 2H), 7.37(dd, J=5.2, 3.2 Hz, 4H), 7.10 (dd, J=5.2, 3.2 Hz, 4H), 4.77 (s, 4H); ¹³CNMR (125 MHz, CDCl₃): δ 188.5, 139.6, 138.7, 126.8, 122.2, 116.9, 57.4;IR (CH₂Cl₂): 3006 (w), 1793 (s), 1437 (w), 1165 (w) cm⁻¹; MS (FAB⁺) m/z335 ((M+H)⁺, 22.8%), 307 (100); HRMS (m/z) calcd for C₃₀H₂₅O₂ 335.1071.Found: 335.1072.

Pentacene and CO Adducts.

The following CO-adducts of pentacene are potential precursors for thegeneration of pure pentacene. The synthetic sequence of compound 13 canbe depicted in Scheme 2, in which the second step (a reduction) wasperformed using sodium borohydride treatment and treatment withphosphorus chloride oxide in pyridine (Py).

Pentacene derivatives with carbonyl bridges include six types, i.e.,types 1, 2, and 11-14. Types 1, 2, and 11 each possess one carbonylbridge; types 12-13 each possess two carbonyl bridges, and type 14possesses three carbonyl bridges. Structures of types 12-13 include bothsyn and anti geometrical isomers (syn isomers are shown), and those oftype 14 include any possible syn/anti geometry (the syn-anti-syn isomeris shown).

In each compound of types 1, 2, and 11-14, there are 14 possiblesubstituents, as indicated by R₁˜R₉ in a general formula (A). In thisformulae X denotes a possible carbonyl bridge (—C(═O)—) across each ofthe six-member ring. There exists at least one carbonyl bridge, but notnecessarily all of them. Groups R₁˜R₉ denote substituents, where R₁˜R₈can be hydrogen, methyl, cyano, methoxy, phenyl, fluoro, chloro, bromoatoms or groups; and R₉ can be hydrogen atom or trimethylsilylalkynylgroup. Specific examples are given as groups 1˜28 in Table 1.

TABLE 1 (Formula A)

Substituents for pentacene derivatives as indicated in general formulaA, B, C, and D. No R₁ R₂ R₃ R₄ R₅ R₆ R₇ R₈ R₉ 1 CH₃ CH₃ H H H H H H H 2CH₃ H CH₃ H H H H H H 3 CH₃ CH₃ CH₃ CH₃ H H H H H 4 CH₃ CH₃ CH₃ CH₃ CH₃CH₃ H H H 5 Cl Cl H H H H H H H 6 Cl H Cl H H H H H H 7 Cl Cl H H Cl H HH H 8 Cl Cl Cl Cl H H H H H 9 Cl Cl Cl Cl Cl H H H H 10 Cl Cl Cl Cl ClCl H H H 11 F F H H H H H H H 12 F H F H H H H H H 13 F F H H F H H H H14 F F F F H H H H H 15 F F F F F H H H H 16 F F F F F F H H H 17 F F FF F F F F H 18 F F F F F F F F F 19 CN CN H H H H H H H 20 CN H CN H H HH H H 21 CN CN CN CN H H H H H 22 OCH₃ OCH₃ H H H H H H H 23 OCH₃ H OCH₃H H H H H H 24 OCH₃ OCH₃ OCH₃ OCH₃ H H H H H 25 OCH₃ H CN H H H H H H 26OCH₃ OCH₃ CN CN H H H H H 27 Ph H Ph H H H H H H 28 H H H H H H H H—C≡C—Si(CH₃)₃

Pentacene can be generated from the dimethyl ketal 15 in acid asevidenced by the absorption spectra shown in FIG. 6. The intermediatediketone 13 cannot be isolated at ambient temperature, yet it can betrapped at a lower temperature. The octachloro-substituted derivativescan be used to yield 1,2,3,4,8,9,10,11-octachloropentacene in a similarway (see FIG. 7). As some fluorinated pentacene derivatives do exhibitFET characteristics, the halogenated analogues are also of potentialusefulness for OTFT.

Ketals that can Release CO₂ and Ethylene.

The thermal stability of compounds 11, 13, and 14 are relatively low,due to the location of CO bridges at the edges of the polycycles.Thermal fragmentation of the corresponding ethylene ketals, e.g.,compound 16, can be achieved directly without going through the ketonederivatives. The solubility of these ketals in organic solvents aresubstantially higher than the corresponding ketones, however, thethermal decomposition temperature is comparatively higher. Nevertheless,the yield of pentacene is good, and the film thus produced exhibitstypical FET characteristics. A single crystal X-ray diffractionstructure of 13 is shown in FIG. 8. The thermogravimetric analysis (TGA)plot (FIG. 9) indicates that the fragmentation happens at about 225° C.

Spiroketals with methyl substituents, e.g., 2,3-butandiol ketal-bridgedprecursor compounds such as 17 and 18, can offer lower fragmentationtemperatures.

Pentacene derivatives with ketal bridges include six types, i.e., types,numbered below as compounds VII˜XII. Types VII˜IX each possess one ketalbridge, types X˜XI each possess two ketal bridges, and type XIIpossesses three ketal bridge. Structures of types X˜XII include both synand anti geometrical isomers.

R₁₀˜R₁₃ denote the substituents in each ketal bridging groups in thestructures VII˜XII, where they can be hydrogen, methyl, ethyl, propyl,butyl, pentyl, hexyl, cyano, methoxy, phenyl, chloro, atoms or groups.R₁₀˜R₁₁ can also be a part of a ring as shown in the following form,where R₁₀˜R₁₁ are expressed as —CH═CH—CH═CH—. In Table 2 it shows someexamples (29˜42) for the ketal substituents R₁₀˜R₁₃.

TABLE 2

Substituents for pentacene derivatives as indicated in the ketal typesVII~XII. No R₁₀ R₁₁ R₁₂ R₁₃ 29 H H H H 30 CH₃ H H H 31 CH₃ CH₃ H H 32CH₃ CH₃ CH₃ H 33 CH₃ CH₃ CH₃ CH₃ 34 CH₃ C₂H₆ H H 35 CH₃ CH₂CH₂CH₃ H H 36CH₃ CH₂CH₂CH₂CH₃ H H 37 C₂H₆ C₂H₆ H H 38 CH₂CH₂CH₂CH₃ H H H 39CH₂CH₂CH₂CH₂CH₃ H H H 40 Ph H H H 41 —CH═CH—CH═CH— H H 42 —CH═CH—CH═CH—CH₃ CH₃

In each compound of types VII˜XII, there are 14 possible substituents,as indicated by R₁˜R₉ in a general formula. In this formulae Y denotes apossible ketal bridge (—OC(R₁₀R₁₁) C(R₁₂R₁₃)O—) across each of thesix-member ring. There exists at least one ketal bridge, but notnecessarily all of them. Groups R₁˜R₉ denote alkyl substituents, whereR₁˜R₈ can be hydrogen, methyl, cyano, methoxy, phenyl, fluoro, chloro,bromo atoms or groups; and R₉ can be hydrogen atom ortrimethylsilylalkynyl group. Specific examples are given as groups 1˜28in Table 1.

Depicted below is the rational design of a series of pentacene COadducts 1, 2, and 11-14, among which syntheses of 1 and 2, and theirphysical properties have been elaborated in separated reports. See K.-Y.Chen, H.-H. Hsieh, C.-C. Wu, J.-J. Hwang, T. J. Chow, Chem. Commun.2007, 1065; H.-H. Huang, H.-H. Hsieh, C.-C. Wu, C.-C. Lin, P.-T. Chou,T.-H. Chuang, Y.-S. Wen, T. J. Chow, Tetrahedron Lett. (2008), 49, 4494;and T.-H. Chuang, H.-H. Hsieh, C.-K. Chen, C. C. Wu, C. C. Lin, P.-T.Chou, T.-H. Chao, T. J. Chow, Org. Lett. (2008), 10, 2869; which arehereby incorporated by reference. Compounds 1 and 2 have successfullyproduced pure pentacene upon heating at 150 and 128° C., respectively.Ibid. The pentacene films thus produced exhibited typical OTFTcharacteristics, i.e. a device made of compound 1 displayed an on/offcurrent ratio about 1.2×10⁵ and field-effect mobility (μ) ca. 0.01cm²V⁻¹s⁻¹. Moreover, for both compounds 1 and 2, it has also been shownthat the CO expulsion can be achieved with the irradiation of light andmay thus be feasible to produce the pentacene films under ambienttemperature to minimize the temperature-dependent annealing process.

Yet the solubility of compound 1 of ˜0.7 mg/mL in methylene chloride orTHF is not quite satisfactory, such that casting processes have to berepeated several times in order to accumulate sufficient materials toprepare a film with required thickness. Likewise, while the solubilityof compound 2 is better than compound 1, it is not believed to besufficiently soluble so as to prepare useful films in a single casting.Thus, still further improvements are presented below.

A pathway for the synthesis of compound 13A (i.e. anti-13) is depictedin Scheme 3. The pentacene skeleton of 22 can be assembled by doubleannulations of anthradiquinone 20 and2,3,4,5-tetrachloro-1,1-dimethoxycyclopentadiene 21. A mixture of synand anti isomers was obtained as yellow solids after being purified bycolumn chromatograph, while the mixture was subjected to the nextsynthetic step. Aromatization of the central anthracene moiety wasaccomplished by a reduction/dehydration sequence, i.e., by reducing thecarbonyl groups to hydroxyls with sodium borohydride, followed by thedehydration with phosphoryl chloride in the presence of pyridine. Thesyn and anti isomers of compound 23 were separated at this stage, andthe geometry of anti-isomer was solved by X-ray crystallography. Thethree aromatic ¹H NMR signals of anti-23 appeared at δ 7.26, 7.87, and8.32. The overall yield of 23 (two isomers) from 20 was about 65%.Compound 23 can be used for the preparation of1,2,3,4,8,9,10,11-octachloropentacene 26, which has been predicted as apotential n-type FET gate material according to Chen et al.,ChemPhysChem 2006, 7, 2003. The chlorine atoms were then stripped off bya reduction of sodium in the presence of t-butanol. However, under sucha condition the central aromatic ring was also reduced, forming compound24. The aromaticity can be regenerated by an oxidation withdichlorodicyanoquinone (DDQ). The ¹H NMR spectrum of 25 is similar tothat of 23, whereas the hydrogen atoms on the double bonds appear at δ6.75. The overall yield of 5-step synthetic routes from 20 to 25 wasabout 45%.

Hydrolysis of the ketal groups of 25 in an attempt to collect diketone13A by standard methods in acidic solutions unfortunately failed.Instead, a complex mixture resulted, probably due to the labile natureof 13A, which decomposed readily at ambient conditions. Pentacene couldbe obtained directly, however, when ketal 25 was treated with aheterogeneous mixture of ferric trichloride and silica gel indichloromethane. Pure pentacene can thus be obtained after filtering offthe solids and dried. A similar hydrolysis can also be applied tooctachloride 23 to yield octachloropentacene 26 as a dark purple solid.

The transformation can be well elaborated via the absorption spectradepicted in FIG. 16. A distinctive strong absorption appeared at 240-320nm is attributed to the ¹A→¹B transition of aromatic chromophore of eachpentacene derivative. For a fair comparison, the spectrum of compound 1(green line) and compound 2 (brown line) at shorter wavelengths areplotted in FIG. 16. The spectra of 23 (red line) and 25 (blue line) arelocated close to each other, with a slight blue shift for the latter.The weak of ¹A→¹L_(a) bands of both 23 and 25 at 300-380 nm nearlyoverlapped with each other. After acidic hydrolysis, the standardpentacene absorption pattern can be clearly identified by both thestrong ¹B band and a very characteristic ¹L_(a) band. The ¹B band ofpentacene appears at 300 nm (black line), and that of 26 at 318 nm (pinkline). The long wavelength ¹L_(a) band of pentacene is located at496-576 nm with characteristic vibronic progressions. The correspondingband of octachloropentacene is at 477-595 nm, again slightly red shiftedwith respect to that of pentacene (inset in FIG. 16).

A slight modification of the ketal structures of 25 resulted inpentacene in good yield. As shown in 25e, the ethylene ketal moietiesdissociate into ethylene and carbon dioxide at an elevated temperature.The TGA curve (red line) indicated that the dissociation started at 215°C., while the percentage of weight loss corresponds properly to thecalculated value of ethylene along with carbon dioxide. The formation ofpentacene was confirmed by its absorption spectrum, which was identicalto an authentic sample. An analogous fragmentation was also found forthe octachloride derivative 23e, while octachloropentacene 26 wasobtained by heating at 285° C. The reaction proceeds quite cleanlyaccording to the weight ratio in TGA profile, as shown in FIG. 17 (blueline).

An OTFT device was made by spin-coating a toluene solution of 25e on thesurface of a SiO₂ gate insulator. An amorphous thin film was formedafter drying, and then was heated at 235° C. for 30 min to convert 25eto pentacene with a good conversion yield. The device exhibited typicalFET characteristics even though the resulting film appeared thin. Theoutput parameters were measured across a source-drain channel with 1 mmwidth and 5 μm length. Plot of drain current (I_(D)) versus drain-sourcevoltage (V_(DS)) at various gate voltages (V_(GS)) is shown in FIG. 15A.The corresponding transfer characteristics, i.e. log(I_(D)) vs. V_(GS)for V_(DS)=−80 V, and ✓I_(D) vs. V_(GS) in the saturation mode is shownin FIG. 15B. The OTFT exhibits an on/off current ratio about 1.7×10³,and an apparent field-effect mobility (μ) about 7.89×10⁻⁶ cm² V⁻¹s⁻¹.The relatively inferior performance of this device is attributable tothe thinness of the pentacene film as mentioned above. The film qualityshould improve by suppressing the rate of pentacene vaporization, or byreducing the annealing temperature.

In summary, the conceptual design and synthesis of a series of solublepentacene precursors have been accomplished. The doubly CO bridgedcompound 13A is not stable enough to sustain at ambient conditions. Uponacidic hydrolysis, the dimethyl ketal derivatives 23 and 25, yieldedpentacenes without isolation of the corresponding ketones. Amorphousthin films can thus be prepared by spin-casting the solutions of 23 and25 in toluene with good solubility; however, heating these films did notyield either pentacene or 1,2,3,4,8,9,10,11-octachloropentacene. Aslight modification on 23 and 25 led to the corresponding ethyleneketals 23e and 25e. TGA analyses of the latter compounds indicated thatthey dissociated at 226 and 290° C., respectively, and generatedpentacenes in high yields. OTFT device made of thin film of 23eexhibited typical FET characteristics.

D. Device Fabrication.

Solid films were made by spin-coating the solutions of precursorcompounds. In some cases multiple spin-coating procedures may be appliedin order to obtain a film of satisfactory thickness. After solvent wasevaporated, a solid web-like film was formed (FIG. 10 shows a film madeof compound 1). Heating the film at an elevated temperature (e.g. 150°C. for compound 1, 128° C. for compound 2, and 230° C. for compound 25e)initiated a fragmentation reaction, thus produced a web-like film ofpure pentacene as shown in FIG. 11 having purplish blue tint.

OTFT devices were made in a standard “bottom contact and bottom gate”manner with channel width 20 cm and length 10 μm were constructed whichhave pentacene films formed from compounds 1 and 2, as well as fromcompound 25e. See FIGS. 12A and 12B. Their characteristics are outlinedin the graphs presented in FIGS. 13A and 13A (FET characteristics fordevice from compound 1), 14A and 14B (FET characteristics for devicefrom compound 2), and 15A and 15B (FET characteristics for device fromcompound 25e).

The results demonstrated in FIGS. 12A-15B indicate that these precursorcompounds are of value for the manufacture of OTFTs.

Experimental Details.

Melting points were taken on a Fargo MP-2D melting-point apparatus andwere not corrected. UV spectra were recorded on a Hewlett-Packard 8453spectrophotometer. Infrared spectra were measured on a Perkin-ElmerL118-F000 FT-IR spectrometer as either thin film or solid dispersion inKBr. Nuclear magnetic resonance spectra were recorded on Bruker AC300and AV500 super-conducting FT NMR spectrometers with all chemical shiftsreported in ppm from tetramethylsilane as an internal standard. Massspectra were obtained on a Joel JMS 700 double focusing spectrometer.Elemental analyses were performed on a Perkin-Elmer 2400 elementalanalyzer. Thermal gravimetric analyses were measured on a Perkin-ElmerPyris 1 thermogravimetric analyzer. Differential scanning calorimetrywas done on a Perkin-Elmer DCS-7 instrument. Column chromatography wascarried out using 230-400 mesh silica gel.

(1) Synthesis of Dimethyl Ketal 23 and 25: Compound 22.

Anthrcene bisquinone (0.33 g, 1.40 mmole) and1,1-dimethoxy-2,3,4,5-tetrachlorocyclopentadiene (1.10 g, 4.2 mmole) wasdissolved in dry toluene under a nitrogen atmosphere. The mixture washeated to reflux for 4 days, then it was cooled to ambient temperature.The solution was filtered, and evaporated to dryness. The yellow solidswere recrystallized from THF:hexane to give a mixture of anti and syngeometrical ⅓ isomers (0.77 g, 73%). ¹H NMR (300 MHz, THF-d₈) signalsassigned to anti isomer: δ 3.55 (s, 6H), 3.70 (s, 6H), 3.89 (s, 4H),8.34 (s, 2H); signals assigned to syn isomer: δ 3.56 (s, 6H), 3.70 (s,6H), 3.88 (2s, 4H), 8.39 (2s, 2H). ¹³C NMR (75 MHz, THF-d₈) signalsassigned to anti isomer: δ 52.63, 53.36, 57.41, 78.72, 112.62, 126.26,130.36, 139.68, 189.40; for syn isomer: δ 52.63, 53.36, 57.41, 78.72,112.56, 126.56, 130.47, 139.68, 189.63. IR (KBr) for mixture: v 3519(m), 3382 (m), 2984 (w), 2956 (w), 2848 (w), 1703 (s), 1601 (m), 1466(m), 1260 (s), 1191 (s), 1133 (m), 1033 (m), 997 (s), 906 (w), 799 (w),547 (w) cm⁻¹. MS (EI) m/z 726.88 ([M-Cl]⁺, 44%). Analysis calculated forC₂₈H₁₈O₈Cl₈: C, 43.90; H, 2.37. Found: C, 43.77; H, 2.26

Compound 23.

Compound 22 (1.00 g, 1.31 mmole) was dissolved in methanol (15 mL) in around bottom flask, which was immersed in a cold water bath. To it wasadded NaBH₄ (0.30 g, 8.0 mmole) slowly to avoid abrupt generation oflarge amount of hydrogen gas. After the addition the mixture was stirredat room temperature for another two hours, it was quenched by theaddition of excess wet THF. The mixture was extracted with THF (50mL×3), filtered, and evaporated in vacuo. The product 22-1 wascrystallized with THF:hexane to give white solids (0.72 g, 71%) as ananti/syn mixture. ¹H NMR (300 MHz, THF-d₈): δ 2.97 (s, 4H), 3.54 (s,6H), 3.55 (s, 6H), 4.88 (d, J=9 Hz, 4H), 4.95 (d, J=9 Hz, 4H), 7.32 (s,2H). ¹³C NMR (125 Mz, THF-d₈): δ 51.99, 53.08, 53.64, 67.61, 77.99,116.71, 128.80, 130.19, 141.92. IR (KBr): v 3550 (m), 3477 (s), 3421(s), 2984 (w), 2949 (w), 2904 (w), 2845 (w), 1638 (m), 1615 (s), 1451(w), 1299 (w), 1213 (w), 1184 (s), 1121 (m), 1002 (s), 902 (w), 777 (w),618 (m), 471 (w) cm⁻¹. Analysis calculated for C₂₈H₂₆O₈Cl₈: C, 43.44; H,3.39. Found: C, 43.38; H, 3.27.

To a round bottom flask containing compound 22-1 (1.50 g, 1.94 mmole) indry pyridine under a nitrogen atmosphere was injected POCl₃ (3.0 g, 20mmole) gradually through a syringe pump. The mixture was heated at 60°C. for 30 hrs for the reaction to complete. It was then cooled to −78°C., and the reactants were quenched by the slow addition of water. Theresulted mixture was extracted several times between CH₂Cl₂ and aqueous20% HCl to remove pyridine. The organic solution was washed withsaturated sodium bicarbonate, dried over anhydrous MgSO₄, filtered, andevaporated in vacuo. The solid residue was purified by silica gelchromatograph eluted with CH₂Cl₂:hexane to give both anti and syn formsof compound 23 (1.12 g, yield 82% for anti and syn combined).Decomposition temperatures for anti: 297° C., and for syn: 271° C. ¹HNMR (400 MHz, CDCl₃) for anti form: δ 3.43 (s, 6H), 3.72 (s, 6H), 7.87(s, 4H), 8.32 (s, 2H); and for syn form: δ 3.42 (s, 6H), 3.72 (s, 6H),7.84 (s, 4H), 8.26 (s, 2H). ¹³C NMR (100 MHz, CDCl₃) for anti form: δ52.42, 53.24, 78.55, 119.70, 121.89, 127.49, 131.21, 135.36, 138.62; andfor syn form: δ 52.39, 53.22, 78.54, 119.67, 121.88, 127.40, 131.15,135.35, 138.58. IR (KBr) for anti form: v 3540 (m), 3510 (m), 3465 (m),3416 (m), 3236 (w), 3053 (w), 2979 (w), 2946 (m), 2841 (m), 1599 (m),1457 (w), 1281 (w), 1195 (s), 1130 (m), 1099 (w), 1013 (m) cm⁻¹; for synform: v 3542 (w), 3508 (w), 3488 (m), 3412 (m), 3234 (w), 2990 (w), 2949(m), 2845 (w), 1638 (w), 1615 (w), 1598 (m), 1455 (w), 1280 (w), 1213(m), 1191 (s), 1151 (m), 1129 (m), 1100 (m) cm⁻¹. MS (FAB⁺) m/z 697.87(M⁺, 5%). Analysis calculated for C₂₈H₁₈O₄Cl₈ anti: C, 47.90; H, 2.58.Found: C, 48.06; H, 2.58; syn: C, 47.90; H, 2.58. Found: C, 47.86; H,2.81.

Compound 25

A three-necked round bottom flask fitted with a refluxing condenser anda nitrogen inlet/outlet was charged with a mixture of anti and synisomers of compound 23 (1.2 g, 1.7 mmole) in dry THF (30 mL) under anitrogen atmosphere. To the flask was added fresh sodium (0.98 g, 43mmole), followed by t-butanol (2.1 g, 28 mmole). The mixture was heatedto reflux for 30 hr. It was then cooled and filtered. The filtrate wasneutralized with saturated sodium bicarbonate, and was extracted severaltimes with CH₂Cl₂. The organic layers were combined, dried overanhydrous MgSO₄, and evaporated in vacuo. A crude product of compound 24was obtained, which was subjected to the next step without furtherpurification.

A crude product of 24 obtained from previous step (0.20 g, 0.47 mmol)was dissolved in 1,4-dioxane (15 mL) under a nitrogen atmosphere. To itwas added dichlorodicyanoquinone (430 mg, 1.9 mmol), and the mixture washeated to 60° C. for 24 hr. The crude product was poured into an aqueoussolution of Na₂S₂O₃ (50 mL) and was stirred for a half hour. The mixturewas extracted with CH₂Cl₂ several times, and the combined organic partswere dried over anhydrous MgSO₄, filtered, and evaporated in vacuo. Thesolid residue was purified by silica gel chromatograph eluted withCH₂Cl₂:hexane to yield 25 in 70% (0.14 g, 0.33 mmol). Decompositiontemperatures for anti is 288° C. and for syn is 252° C. ¹H NMR (300 MHz,CDCl₃) for anti: δ 3.11 (s, 6H), 3.31 (s, 6H), 4.09 (d, J=3 Hz, 4H),6.66 (t, J=3 Hz, 4H), 7.66 (s, 4H), 8.08 (s, 2H); and for syn: δ 3.11(s, 6H), 3.32 (s, 6H), 4.11 (d, J=3 Hz, 4H), 6.67 (t, J=3 Hz, 4H), 7.68(s, 4H), 8.10 (s, 2H). ¹³C NMR (75 MHz, CDCl₃) for anti: δ 50.77, 52.02,53.41, 119.53, 125.42, 126.66, 130.73, 137.92, 143.07; and for syn: δ50.74, 52.01, 53.40, 119.51, 125.41, 126.65, 130.71, 137.88, 143.09. MS(EI) m/z 426.18 (M⁺, 100%). Analysis calculated for C₂₈H₂₆O₄: C, 78.85;H, 6.14. Found: C, 78.47; H, 6.23.

(2) Synthesis of Ethylene Glycol Ketal 23e and 25e

The procedures for the preparations of ethylene glycol ketal 23e and 25eare basically the same as those for the preparation of 23e and 25e asdepicted in the following scheme.

Compound 22e.

A similar procedure for the preparation of compound 22 was conducted. Acombined yield of syn and anti isomers of 22e was 80%. ¹H NMR (300 MHz,THF-d₈): δ 3.88 (s, 4H), 4.23 (t, J=6 Hz, 4H), 4.35 (t, J=6 Hz, 4H),8.42 (s, 2H). ¹³C NMR (75 MHz, THF-d₈): δ 57.34, 68.20, 69.40, 77.48,121.56, 126.69, 130.11, 139.69, 189.42. IR (KBr): v 3603 (w), 3523 (m),3442 (m), 3346 (m), 3208 (w), 2974 (w), 2908 (w), 1704 (s), 1628 (w),1593 (s), 1532 (w), 1472 (w), 1417 (w), 1256 (s), 1215 (s), 1164 (w),1122 (m), 1096 (w), 1027 (m), 1002 (m) cm⁻¹. MS (FAB⁺): m/z 758.82((M+H)⁺, 30%). Analysis calculated for C₂₈H₁₄O₈Cl₈: C, 44.13; H, 1.85.Found: C, 44.34; H, 2.28.

Compound 23e.

Compound 22e was first reduced by NaBH₄ to give 22e-1 according to thesame procedure for the preparation of 22-1. The combined yield of synand anti isomers of 22e-1 was 83%. ¹H NMR (400 MHz, THF-d₈): δ 2.89 (s,4H), 4.13 (t, J=6 Hz, 4H), 4.20 (t, J=6 Hz, 4H), 4.88 (d, J=8 Hz, 4H),5.04 (d, J=8 Hz, 4H), 7.34 (s, 2H). ¹³C NMR (125 MHz, THF-d₈): δ 53.70,67.25, 67.66, 68.89, 76.63, 125.28, 128.73, 129.78, 141.71. IR (KBr): v3663 (w), 3536 (w), 3416 (w), 3301 (s), 2934 (w), 2906 (m), 1660 (w),1630 (w), 1601 (m), 1472 (w), 1300 (w), 1220 (s), 1186 (w), 1160 (w),1114 (s), 1032 (s), 998 (s), 909 (m) cm⁻¹; MS (FAB⁺) m/z 765.88 (M⁺,4%). Analysis calculated for C₂₈H₂₂O₈Cl₈: C, 43.67; H, 2.88. Found: C,43.76; H, 3.01.

Compound 22e-1 was dehydrated to yield 23e in a similar way to theprocedure of 22-1 to 23. The yield of 23e was 83%. ¹H NMR (400 MHz,CDCl₃) for anti: δ 4.22˜4.24 (m, 4H), 4.33˜4.35 (m, 4H), 7.91 (s, 4H),8.40 (s, 2H); and for syn: δ 4.24˜4.28 (m, 4H), 4.35˜4.39 (m, 4H), 7.84(s, 4H), 8.19 (s, 2H). ¹³C NMR (100 MHz, CDCl₃) for anti: δ 67.41,67.99, 77.31, 119.93, 127.66, 130.25, 131.12, 134.58, 137.63; and forsyn: δ 67.42, 68.00, 77.21, 119.93, 127.62, 130.21, 131.05, 134.57,137.56. IR (KBr) for anti: v 3549 (w), 3472 (m), 3416 (m), 3234 (w),2990 (w), 2905 (w), 1638 (w), 1616 (w), 1591 (w), 1282 (w), 1241 (m),1216 (s), 1166 (w), 1121 (m) cm⁻¹; for syn: v 3411 (m), 3060 (w), 2990(w), 2904 (m), 1662 (w), 1591 (m), 1475 (w), 1336 (w), 1282 (w), 1218(s), 1166 (m), 1121 (s), 1030 (m) cm⁻¹; MS (FAB⁺) m/z 694.85 ((M+H)⁺,19%). Analysis calculated for compound C₂₈H₁₄O₄Cl₈: C, 48.18; H, 2.02.Found: C, 48.21; H, 2.03.

Compound 25e.

The preparation of 25e from 23e via 24e was similar to that of 25 from23. Decomposition temperatures were 238° C. for anti and 242° C. for synisomers. Physical data of 25e: ¹H NMR (300 MHz, CDCl₃) for anti: δ3.93˜3.99 (m, 12H), 6.75 (t, J=2 Hz, 4H), 7.73 (s, 4H), 8.13 (s, 2H);for syn: δ 3.92˜3.94 (m, 8H), 3.98˜4.00 (m, 4H), 6.75 (t, J=2 Hz, 4H),7.72 (s, 4H), 8.13 (s, 2H) ¹³C NMR (75 MHz, CDCl₃) for anti: δ 55.13,64.99, 65.53, 119.71, 125.76, 130.71, 132.76, 137.92, 142.25; and forsyn: δ 55.14, 65.01, 65.58, 119.73, 125.75, 130.73, 132.78, 137.97,142.28. MS (FAB⁺) m/z 423.16 ((M+H)⁺, 100%). Analysis calculated forC₂₈H₂₂O₄: C, 78.85; H, 6.14. Found: C, 79.22; H, 5.70.

Tetracene Derivatives.

Tetracene has also been approved as a useful p-type gate material forOTFT applications. Its soluble precursors containing carbonyl bridgeshave been prepared in this work as well. As indicated in the generalformula C, there are 12 possible substituents indicated as R₁˜R₈, whichdenote substituents such as hydrogen, methyl, cyano, methoxy, phenyl,fluoro, chloro, bromo atoms or groups. Specific examples are given asR₁˜R₈ of numbers 1˜28 in Table 1.

A carbonyl adduct of tetrocene, i.e., compound 34, was also synthesized.This compound can extrude a molecule of CO, either upon heating or underlight irradiation, to generate highly pure tetracene. The syntheticscheme is demonstrated according to the following series of reactions:

Compound 32.

To a two-necked round bottom flask fitted with a refluxing condensercontaining compound 31 (2.56 g, 7.5 mmole) in dry THF (150 mL) was addedfresh sodium (2.60 g, 113 mmole) under a nitrogen atmosphere. Themixture was stirred at ambient temperature for 20 min, then t-butanol(4.45 g, 60 mmole) was added. The resulting mixture was heated to refluxfor 20 hr. It was cooled, and the remaining sodium was filtered off. Thesolution was neutralized with saturated sodium bicarbonate solution, andwas extracted several times with dichloromethane. The organic portionswere combined, dried over anhydrous MgSO₄, and evaporated in vacuo. Theresidue was purified by silica gel chromatograph eluted withhexane/ethyl acetate to give compound 32 as white solids (0.99 g, 65%).¹H NMR (400 MHz, CDCl₃): δ 3.06 (s, 3H), 3.26 (s, 3H), 4.02 (t, J=2.2Hz, 2H), 6.72 (t, J=2.2 Hz, 2H), 7.02 (dd, J=3.1, 5.1 Hz, 2H), 7.24 (dd,J=3.1, 5.1 Hz, 2H).

Compound 33:

In a round bottom flask compound 32 (100 mg, 0.49 mmole) was dissolvedin dry DMF (15 mL) under a nitrogen atmosphere, then it was heated to90-100° C. A mixture of 1,2-bis(dibromomethyl)benzene (1.04 g, 2.5mmole) and sodium iodide (1.48 g, 9.9 mmole) was added gradually intothe above solution. It was heated for a period of 48 hr, then wascooled. The resulting mixture was poured into an aqueous solution of 20%sodium thiosulfate, then was extracted several times withdichloromethane. The organic layers were combined, washed twice withdistilled water, dried over anhydrous MgSO₄, and evaporated in vacuo.The brown solids were purified with silica gel chromatograph eluted withhexane/ethyl acetate to gave compound 33 which appeared as pale yellowsolids (21 mg, 14%). ¹H NMR (400 MHz, CDCl₃): δ 3.20 (s, 3H), 3.23 (s,3H), 4.58 (s, 2H), 7.04 (m, 2H), 7.31 (m, 2H), 7.38 (m, 2H), 7.66 (s,2H), 7.67 (s, 2H).

Compound 34.

To a round bottom flask containing acetone (10 mL) was added compound 33(21 mg, 0.070 mmole), sodium iodide (104 mg, 0.70 mmole), and ceriumchloride hydrate (258 mg, 0.70 mmole), followed by a small amount ofconcentrated hydrochloric acid (0.2 mL). The mixture was stirred at roomtemperature for 3 days. The solution was neutralized by saturated sodiumbicarbonate solution, and was extracted several times withdichloromethane. The organic portions were combined, washed with brine,dried over anhydrous MgSO₄, and evaporated in vacuo. Compound 34 wascollected as pale yellow solids (16.1 mg, 91%). ¹H NMR (400 MHz, CDCl₃):δ 4.88 (s, 2H), 7.18-7.20 (m, 2H), 7.44-7.46 (m, 2H), 7.48-7.50 (m, 2H),7.79-7.81 (m, 2H), 7.89 (s, 2H).

Anthracene Derivatives.

Anthracene with a CO bridging across 4a,8a-positions, i.e., compoundexpressed by formula D where R₁═H, is known. The substituents R₁₄ can bemodified to certain aromatic groups or aryl substituted ethenyl groups.This group of compounds can be expressed by a general formula D, inwhich R₁₄ is a kind of substituent at the diagonal positions.

TABLE 3 (Formula D)

Substituents for R₁₄ as indicated in formula D for anthracenederivatives.

While anthracene derivatives 35, 36, 37 and 38 have been shown toexhibit good p-type OTFT characteristics, they all have undesirably lowsolubility in organic solvents.

It has been known that the carbonyl adduct of anthracene can undergo COelimination in refluxed benzene, as indicated in the following scheme.

The following carbonyl adducts 39-42 can be synthesized readily, andshould exhibit similar CO expulsion reaction to generate thecorresponding anthracene compounds 35-38 at 130° C.

Cycloaddition of 3-bromofuran and compound 43 was accomplished byheating at 130° C. in a sealed tube. A 1:1 ratio of syn 44 and anti 45isomers were obtained. The anti isomer 45 can be substituted by aselected aromatic group through a palladium coupling reaction. Theproduct 46 can be dehydrated upon acidic treatment to yield compound 47.Ozonolysis of compound 47, shown below, will give compound 41. Otherderivatives 39-42 can be prepared by similar reaction sequences.

Compound 45. ¹H NMR spectrum of compound 45 (400 MHz, CDCl₃) is asfollows: δ□1.50 (s, 3H), 1.65 (s, 3H), 2.41 (m, 1H), 2.48 (m, 1H), 3.36(s, 1H), 3.66 (s, 1H), 4.54 (m, 1H), 4.80 (m, 1H), 6.15 (s, 1H), 6.92(d, J=8 Hz, 1H), 7.11 (d, J=8 Hz, 1H), 7.21 (s, 1H). ¹³C NMR (100 MHz,CDCl₃): δ 19.85, 22.84, 43.88, 44.20, 47.90, 48.69, 81.92, 84.49,116.66, 118.74, 121.35, 123.15, 125.72, 128.18, 131.13, 144.39, 148.08,151.26.

CO Expulsion by Both Thermal and Photochemical Methods.

All the carbonyl adducts reported herein are reactive either thermallyof photochemically. Thermal fragmentation promoted by heating can betraced by TGA or DSC scans. FIGS. 3A and 3B clearly show the productionof pentacene from compounds 1 and 2 upon irradiation with light.

An advantage of photo over thermal methods for CO expulsion is that theformer can be used on photo-lithography. A patterned thin film ofpentacene can be made by first spin-coating a thin film of theprecursors, then the film is covered by a mask to prevent light frompassing through. The film with mask is placed under light, while theareas without mask will be converted into insoluble pentacene. After themask is removed, and the chemical film is washed with solvent todissolve the un-reacted precursors, a film of pentacene with a desiredpattern of the mask is obtained.

What is claimed is:
 1. A tetracene derivative comprising formula C:

wherein R₁-R₈ are independently selected from the group consisting ofhydrogen, methyl, cyano, methoxy, phenyl, fluoro, chloro, and bromo, andwherein at least two of R₁-R₈ are not hydrogen.
 2. A method of forming afilm on a substrate comprising: applying a composition to form a film onthe substrate, the composition comprising a tetracene derivativeaccording to claim 1; and expelling volatile units of CO or CO₂ from thefilm by at least one of: thermally treating the film and photochemicallytreating the film.
 3. The method of claim 2, wherein the applyingcomprises spin-coating to form a film.
 4. The method of claim 2, whereinthe composition further comprises at least one organic solvent.
 5. Themethod of claim 2, further comprising masking at least a portion of thefilm before expelling volatile units of CO or CO₂ from the film byphotochemically treating the film.
 6. The method of claim 5, furthercomprising removing the mask and washing the film with a solvent afterphotochemically treating the film.
 7. A tetracene derivative accordingto claim 1, wherein the tetracene derivative comprises R₁-R₈ as selectedfrom the group consisting of numbers 1-27 according to the table: No. R₁R₂ R₃ R₄ R₅ R₆ R₇ R₈ 1 CH₃ CH₃ H H H H H H 2 CH₃ H CH₃ H H H H H 3 CH₃CH₃ CH₃ CH₃ H H H H 4 CH₃ CH₃ CH₃ CH₃ CH₃ CH₃ H H 5 Cl Cl H H H H H H 6Cl H Cl H H H H H 7 Cl Cl H H Cl H H H 8 Cl Cl Cl Cl H H H H 9 Cl Cl ClCl Cl H H H 10 Cl Cl Cl Cl Cl Cl H H 11 F F H H H H H H 12 F H F H H H HH 13 F F H H F H H H 14 F F F F H H H H 15 F F F F F H H H 16 F F F F FF H H 17 F F F F F F F F 18 F F F F F F F F 19 CN CN H H H H H H 20 CN HCN H H H H H 21 CN CN CN CN H H H H 22 OCH₃ OCH₃ H H H H H H 23 OCH₃ HOCH₃ H H H H H 24 OCH₃ OCH₃ OCH₃ OCH₃ H H H H 25 OCH₃ H CN H H H H H 26OCH₃ OCH₃ CN CN H H H H 27 Ph H Ph H H H H H.